The present invention is concerned with a gumming solution for use in the burning-in of irradiated and developed offset-printing plates, and with a process for offset printing plate production in which the aforesaid gumming solution is applied between the stages of developing and burning-in the printing plate.
Offset-printing plates usually comprise (a) a planar support material comprised of a metal and/or a plastic material, the surfaces of which can be modified, for example, by graining (etching) and/or by an anodic oxidation treatment, and (b) at least one radiation-sensitive (photosensitive) coating (reproduction layer). In the production of a printing plate, in which the basic operating steps include irradiation, development, and gumming (desensitizing), a burning-in step is additionally carried out between the last two steps, especially with positive-working reproduction layers, although burning-in is also possible with negative-working reproduction layers.
For positive-working reproduction layers, the procedure can be explained as follows: during the imagewise irradiation (exposure) step, those portions of the reproduction layer struck by electromagnetic energy (light) are rendered soluble, while the non-irradiated portions remain insoluble, relative to the developer which is to be used. By removing the irradiated portions of the layer in the developing step, the non-image areas, which are hydrophilic in the subsequent printing with the printing plate, appear on the printing plate surface, but the non-irradiated portions of the layer remain on the support and form the image areas which are ink-receptive in the subsequent printing process.
In practice, it has been found that heating of the irradiated and developed printing plate to a temperature exceeding 80.degree. C., particularly to more than about 150.degree. C., depending on the type of the support material and/or of the reproduction layer, with heat being applied either on the side of the reproduction layer or on the reverse side, can result in larger print runs obtained from the printing plate thus treated. This procedure is called "burning-in". By burning-in, curing of the image areas is achieved, to the extent that the image areas, for example, are rendered insoluble in the usual organic solvents and become resistant to the action of chemicals which may be present in the printing ink or in the dampening solution used in the subsequent printing process.
In the burning-in process, constituents of the image areas (i.e., of the reproduction layer), such as the binder constituents, can be deposited in the non-image areas, adversely influencing the hydrophilic character of these areas. This can result, for example, in scumming, i.e., the picking-up of printing ink by the non-image areas. In the initial phase of using a burning-in step, it has therefore been necessary to remove decontamination from the non-image areas by means of relatively caustic solutions. So-called "burning-in auxiliaries" have been developed which at least facilitate this post-cleaning operation. But as a rule, it has still been necessary to affect an additional cleaning of the non-image areas with water, before the actual printing process. Burning-in auxiliaries which are known to the art include the following:
In German Patent No. 2,318,286 (corresponding to U.S. Pat. No. 3,745,011), a process for the production of negative resist images is described in which a photoresist is exposed, developed and then treated with a solution of a cycloaliphatic, heterocyclic, or aromatic polycarboxylic acid, or with a solution of an aromic sulfonic acid, before carrying out a burning-in process. The photosensitive layers of the photoresists which are employed contain natural proteins, such as fish glue, and ammonium bichromate. This treatment before burning-in is intended to (a) lower the burning-in temperature, (b) increase the resistance of the resist image to etchants, and (c) improve the stripability of the resist image or of the applied photoresist, respectively. The acids that are used include, for example, p-toluene sulfonic acid, 2-naphthalene sulfonic acid, naphthalene-1,8-disulfonic acid, benzene sulfonic acid, and 1,3-benzene-disulfonic acid, and are preferably employed in the form 1% to 25% strength solutions. As the support material for coating with a photoresist, steel is exclusively used. Printing plates and the specific burning-in problems encountered in this field of application are not mentioned in this publication.
German Patent No. 2,530,422 (corresponding to British Published Application Ser. No. 1,513,368) disclosed a process for manufacturing printing plates, boards for printed circuits, integrated circuits and the like. In the process, the exposed and developed supports are provided with a protective layer which is applied at least to the non-image areas before burning-in. During burning-in, this protective layer remains on the support and can be removed with water after burning-in, together with a deposit which has settled on the protective layers in the burning-in process. The protective layer is applied as an aqueous solution about 2.5% to 45% strength of one of the following compounds: sodium dodecyl phenoxy benzene disulfonate, a sodium salt of an alkylated naphthalene sulfonic acid, a disodium salt of methylene di-(naphthalene sulfonic acid), sodium dodecyl benzene sulfonate, a sodium salt of a sulfonated alkyldiphenyl oxide, ammonium perfluoroalkyl sulfonate, potassium perfluoroalkyl sulfonate, sodium dioctyl sulfosuccinate, sodium-di(methylamyl)-sulfosuccinate, and LiNO.sub.3.
In British Published Application No. 1,575,200, which is based on the above-mentioned German patent publication, the aqueous solution used to form the protective layer is, before burning-in, admixed with a water-soluble hydrocolloid selected from the group consisting of gum arabic, sodium carboxymethyl cellulose, and alginate, in addition to one of the above-indicated compounds. The composition employed should comprise from 85% to 98% by volume of one of the compounds disclosed in British Published Application No. 1,513,368 and from 2% to 15% by volume of one of the hydrocolloids and is used in the form of a 10% to 50% by volume strength aqueous solution. After burning-in, the printing plates are washed with water and can then be mounted in a printing machine. The disclosed composition could be designated either a "gumming solution for burning-in" or a "desensitizing agent for burning-in", since it acts as a "burning-in auxiliary" and also as a "gumming solution".
The burning-in auxiliary according to German Patent No. 2,625,336 (corresponding to U.S. Pat. No. 4,063,507) comprises boric acid or a borate, and is used in a solid form or, preferably, as a 3% to 10% strength aqueous solution. According to German Offenlegungsschrift No. 2,626,473 (corresponding to British Published Application No. 1,555,233), it is also possible to use, among others, various hydrophilic polymers, such as gum arabic, dextrin, polyvinyl alcohol, cellulose ethers, homopolymers and copolymers based on acrylic acid or methacrylic acid, a copolymer of alkyl acrylate and vinyl methyl acetamide, and even salt of organic acids, for example, the sodium salt of anthraquinone-2,7-disulfonic acid, the sodium salt of naphthalene-1,3,5-trisulfonic acid, the sodium salt of 1-naphthylamine-4,6,8-trisulfonic acid, and the sodium salt of 1-naphthol-3,6-8-trisulfonic acid, in lieu of the compounds (burning-in auxiliaries) which have been described above as being suitable for use in the burning-in of printing plates. The compounds are used in aqueous solutions at concentrations ranging from 0.1% up to saturation.
As further burning-in auxiliaries, carboxyl-group containing amines, for example, ethylene diamine tetraacetic acid, nitrilo triacetic acid and the salts thereof, are specified in European Patent Application No. 0,043,991 (corresponding to U.S. Pat. No. 4,355,096), and salts or salt-like reaction products of naphthalene, formaldehyde and sulfuric acid are mentioned in U.S. Pat. No. 4,191,570.
All burning-in auxiliaries which are known in the art still present disadvantages. They can, for example, cause a reduction in the oleophilic properties of the image areas, lead to staining on the printing-plate surface and, particularly with insufficient post-cleaning with water or aqueous developer solutions, the production of irregularities in the water/ink balance in the subsequent printing process. In many descriptions concerning the use of known burning-in auxiliaries, attention is specifically drawn to these problems, which are mentioned as being solvable, for example, by rubbing dry the printing-plate surface before burning-in, by thoroughly rewashing, or even by carrying out a hydrophilizing post-treatment/cleaning using an aqueous solution of a hydrophilic polymer like polyvinyl phosphonic acid [see, for example, European Patent Application No. 0,012,956 (corresponding to U.S. Pat. No. 4,265,999)].
Special hydrophilizing post-treatments of this kind, and even one of the customary gumming (desensitizing) post-treatments, are feasible, but they require an additional working step in the production and use of a printing plate. A combination of the two steps in a single burning-in and gumming treatment is described in the above-mentioned British Published Application No. 1,575,200. This treatment still results in a high consumption of paper in the printing machine, however, since without additional, intermediate cleaning the printing plate takes too long to run clean (see also the comparative examples described below).